Fill a volume with spheres from a particles generator
If I am correct that this is the effect that you are trying to achieve, the most important thing is enabling collision on the volume sphere, not rigid body simulation.

A quick visual walk-through of what I did...

1. Add three spheres


Volume sphere
Emitter sphere
Particle sphere




Note: the max draw type of the volume sphere was set to wire to make it easier to see.

2. Setup particle system on the emitter


Give the particles a long lifetime (like 250+)
Emit them from the volume of the mesh
Turn off gravity effects
Set the render mode to object with the particle sphere selected




3. Enable particle collision on the volume sphere



Note: the default settings work fine if you don't want particle damping/killing.